This is only a preview of the September 2000 issue of Silicon Chip. You can view 36 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build A Swimming Pool Alarm":
Items relevant to "8-Channel PC Relay Board":
Items relevant to "Fuel Mixture Display For Cars, Pt.1":
Articles in this series:
Purchase a printed copy of this issue for $10.00. |
Drowning is one of the primary causes of accidental death
in children under five. SILICON CHIP would love to see that
statistic eliminated – and this simple, effective pool alarm
could assist in that aim. Don’t wait until summer: build it NOW!
Features
Compact, battery operated
Free-floating unit
Loud siren sounds upon
sudden pool water mo
vement
Reduced sensitivity to win
d
movements and side-of-p
ool
collisions
Splashproof and rain proof
On/Off switch for pool use
Test switch lowers siren vol
ume
50 second alarm
Swimming
Pool
Alarm
by JOHN CLARKE
12 Silicon Chip
W
hile properly designed and
maintained pool fences are
the primary line of defence
in preventing young children falling
into a pool, they are not enough.
Indeed, we speak from first-hand,
recent experience: Georgia, the
18-month-old “model” used in our
photographs, was found leaning over
the edge of the pool (after that same
ball) shortly after the photographs
were taken. The reason – the gate had
not latched properly.
We shudder to think what might
have been if we weren’t close by.
Needless to say, the gate latch has
now been fixed … and we now have
a SILICON CHIP pool alarm floating in
the water.
The problem is, children are very
resourceful when it comes to getting
to a pool: the smallest gap in the fence;
a box or chair left where it can be
climbed on (or dragged to the fence);
even a dog digging a tiny hole under
the fence (if a dog can squeeze through
it, a small child can often do so too…)
So it is unwise to be complacent
about pool safety, even if you think
your pool fence is impenetrable.
Some people install a closed-circuit
TV system to monitor their pool area,
with the screen in, say, the kitchen.
That’s pretty good to keep watch
over the kids while they’re in the
pool and mum, for example, is inside.
But what happens when she’s not in
the kitchen?
And even the best TV monitoring
system is useless when you’re away
and the neighbour’s toddler finds his
or her way into your backyard…
A much better form of protection
is to use a device which can detect
someone actually falling into the
pool – and then screams its head off.
Of course, it would be better to detect them before they fall in but that’s
It’s like an insurance policy: you never know when you need it but you’ll always be grateful that you had it if it’s ever really needed!
getting even more difficult!
One way to detect someone falling
into the pool is to sense any small
change in the water level and set off
an alarm if the change in level matches certain parameters – for example,
changes caused by wind or filter
Specifications
...... 330uA at 6V
Battery current drain.......
.... >6 months
Expected battery life............
ally 50 seconds
Alarm duration..........typic
typically .01g
Movement sensitivity..........
action need to be rejected).
The SILICON CHIP Swimming Pool
Alarm is based on this principle.
It is fully self-contained and battery
operated. There is a small plastic box
which is simply left to float on top of
the pool surface (you could loosely
tether it if necessary). Inside is a
sensor which detects small, though
rapid, changes in the pool water level
as would happen when someone falls
in. On detecting this change, a siren
sounds.
It’s designed to be left on all the
time, except of course when the
pool is being used. A waterproof on/
off switch is provided to allow it to
be removed from the pool without
sounding, and there is another switch
which tests the alarm (at reduced volume) to periodically check the battery.
Detection
Detecting a small child’s body
entering the pool is rather difficult.
The monitoring must attempt to exclude normal pool movements caused
by wind or filter operation but still
detect changes in water level.
Even this is not foolproof: if the
Fig.1: the block diagram of the SILICON CHIP Swimming Pool Alarm. The weighted piezo sensor detects water disturbance in the pool.
SEPTEMBER 2000 13
Fig.1: the low frequency signal from the piezo sensor is amplified to trigger the siren driver.
person climbs slowly down the ladder
the rate of change in water level might
be virtually nonexistent.
By contrast, anyone actually falling
into the pool will usually make quite
a splash with lots of movement of the
pool surface.
Even a child overbalancing while
leaning over the pool edge to retrieve
a toy or ball (by far the most common
scenario) will make large ripples on
the pool surface.
Water level change detection is a
compromise between sufficient sensitivity for the purpose intended while
rejecting normal pool movement due
to wind, etc.
As such, it may produce false alarm
signals on a windy day.
What we are saying is that this
method of sensing water level change
can never be 100% reliable but it is
about as reliable as can be achieved
(within reason).
No pool alarm can give you absolute assurance – it is very much your
second line of defence. Always ensure
the pool fence and gates are in perfect
14 Silicon Chip
order and remain vigilant while ever
kiddies are around.
Block diagram
The block diagram of the Swimming Pool Alarm is shown on Fig.1.
The sensor itself consists of a piezo
element which supports a weight. The
piezo element is attached to a floating
box which floats on the swimming
pool surface.
Any upwards-movement of water
will cause the floating box to rise,
pushing against the piezo which
doesn’t move as quickly due to the
inertia of the attached weight. When
this occurs, the piezo element generates a small voltage output.
Note that a downward movement
of the box will not usually cause decompression of the element. This is
because the floating box drops with
gravity at the same rate as the mass.
The signal from the piezo detector
is amplified by IC1a and filtered so
that only frequencies below about
2Hz pass through. The amplifier has a
gain of 33 for frequencies below 2Hz.
As the mass on the piezo element
also damps out any fast movement
(again due to inertia), it reduces the
high frequency response of the piezo
element. Thus the output from the
filter only changes for slower movements.
The signal is squared up by the following Schmitt trigger (IC1b) and has
an adjustable threshold to allow setting the sensitivity to pool movement.
The Schmitt trigger output is a low
frequency square wave which changes
with the piezo detector output. The
signal drives a charge pump which
requires at least two pulses from the
Schmitt trigger before the output from
the charge pump is low enough to
trigger the following timer. This requirement before triggering the timer
reduces the likelihood of false alarms.
The timer produces a high signal
for about 50 seconds which drives
the siren driver (Q1) and siren. The
siren should be sufficiently loud to
attract attention.
The circuit is housed in a sealed box
to prevent water getting in. However,
the siren must be exposed to the outside air so that it can be heard. It also
needs to be made as loud as possible
to attract attention. This is done by
feeding the siren into a tuned port,
covered to make it splashproof.
Actual dimensions of the port and
cover are fairly critical to maximise the sound output level. Simply
placing an untuned cover over the
siren outlet would severely muffle
the volume.
Fortunately, we found a couple
of easily-obtainable items made a
near-perfect port: the flange or front
section from a standard bayonet-cap
light fitting (the bit that screws on to
the actual lampholder which holds a
lampshade or diffuser in place) and
half a table-tennis (or ping-pong) ball!
Circuit
Use this component overlay with the photograph below as a reference while
building the Pool Alarm and you shouldn’t go wrong. The two 25mm M3
screws don’t actually hold anything – they're there (with the four small “L”shaped brackets not shown in the photo) to stop the battery holders slopping
around in the case.
Fig.2 shows the circuit for the
Swimming Pool Alarm.
Signal from the piezo transducer
is connected to the low pass filter,
comprising IC1a and the associated
resistors and capacitors. The 100kΩ
resistors and 1µF capacitors set the
low pass filter at 2.3Hz, while the
3.3MΩ feedback resistor and .015µF
capacitor set the gain at 33 times at
or below the 2.3Hz rolloff frequency.
IC1a is biased at 1/2 supply (+3V)
at pin 3 by the 1MΩ voltage divider
resistors connected across the supply.
This half supply is decoupled with a
100µF capacitor.
The output of IC1a is also at 1/2
supply and this drives a 2.2kΩ resistor
decoupled with a 470µF capacitor.
The voltage across the 470µF capacitor is therefore at 3V (1/2 supply)
and the resistor and capacitor form a
low pass filter to reject signals above
0.15Hz.
Hysteresis for the Schmitt trigger
(IC1b) is set by the ratio of resistance
between the 3V supply and pin 5 and
the resistance between pins 5 & 7.
Thus the hysteresis can be varied
from about 13mV when VR1 is wound
with its wiper closest to the 2.2kΩ resistor and around 300mV when VR1’s
wiper is closest to the 1MΩ resistor.
The output of the Schmitt trigger
is used to drive a “charge pump”
consisting of diodes D1 & D2 and
capacitors C1 & C2. These produce a
voltage negative with respect to the
+6V line across capacitor C2 whenever IC1b's output is toggling (ie, the
circuit is sensing water disturbance).
SEPTEMBER 2000 15
The voltage across C2 is fed to pin 2,
the trigger input of timer IC2.
IC2 is triggered when its pin 2 goes
below one third of the supply voltage,
or 2V. When triggered, the 47µF capacitor begins charging via the 1MΩ
resistor and the pin 3 output goes
high and drives transistor Q1’s base
via the 2.2kΩ resistor. This transistor
drives the siren.
The siren can be driven directly via
the 6V supply or via the 10kΩ resistor
connecting to the 6V supply for a reduced output level (for testing). This
is selected using switch S2.
The output of IC2 (pin 3 ) stays
high until the 47µF capacitor at pin
6 reaches two thirds of the supply
voltage. The pin 3 output then goes
low and the capacitor is discharged
via the pin 7 output and 10kΩ resistor. The time duration for the alarm
is around 50 seconds.
When power is first switched on,
the reset input of IC2, pin 4, is held
low via the 10µF capacitor to prevent
the timer from being triggered by
IC1b. After about a second the reset
pin voltage reaches about 1V due to
the 10µF capacitor being charged via
the 560kΩ resistor and then the timer
can be triggered.
Construction
The Swimming Pool Alarm is
constructed using a PC board coded
03109001 and measuring 89 x 80mm.
This is housed in a sealed plastic enclosure measuring 115 x 90 x 55mm.
It is important to use this case, not
the plastic project boxes we normally
use, as this one has an integral gasket
in the lid ensuring it is waterproof.
A front panel label measuring 108 x
85mm attaches to the lid of the case.
We used a flange cover from a bayonet light socket to cover the piezo
siren outlet and made a splashproof
hood for it by cutting a table tennis
ball in half. The front panel switches
were also waterproofed with rubber
hoods.
Begin construction by checking the
PC board for shorts or breaks in the
tracks. Also check the hole sizes for
fit, especially for the PC stakes, the
3mm screw holes required for the
piezo transducer and AA cell holder
locating screws. The four corner holes
need to be 3mm in diameter.
Insert and solder in the resistors
and diodes D1 and D2. Use the accompanying resistor colour code table as
16 Silicon Chip
We used the flange from a standard
240V light fitting (at left in photo
above) to form the “tuned port”
cover over the piezo buzzer. This was
capped with half a ping-pong ball.
The diagram at right shows how the
various parts are assembled.
a guide to selecting the correct values
for each position. If in doubt use your
multimeter to verify values. Note that
the 560kΩ resistor is mounted on its
end as shown. Ensure that D1 and
D2 are inserted the right way around.
When mounting IC1 and IC2 take
Parts List – Swimming Pool Alarm
1 PC board coded 03109001, 89
x 80mm
1 sealed ABS enclosure, 115 x
90 x 55mm (Jaycar HB-6126
or DSE H-2863 or equiv.)
1 front panel label 111 x 87mm
2 2 x AA cell holders
4 AA cells
1 dual sound piezo buzzer
(Jaycar AB-3456 or equiv.)
1 piezo audio transducer 30mm
diameter
2 SPDT toggle switches (S1,S2)
2 waterproof boots or hoods for
toggle switches
1 brass or lead cylinder 15mm
OD x 19mm*
1 flange cover from a mains
bayonet light socket or line
socket (tapered from 36mm to
32mm over 35mm length.
1 38mm diameter table tennis
ball
2 40mm lengths of 1.25mm diameter cold drawn brass wire
4 right angle brackets 7 x 9 x
10mm wide
4 M3 x 6mm screws
2 M3 x 10mm screws
2 M3 x 25mm screws
4 M3 nuts
11 PC stakes
1 200mm length of red hookup
wire
1 200mm length of black hookup
wire
Semiconductors
1 TL062 dual low power op amp
(IC1)
1 7555, LMC555CN CMOS 555
timer (IC2)
1 BC338 NPN transistor (Q1)
2 1N914, 1N4148 switching
diodes (D1,D2)
Capacitors
2 470µF 16VW PC electrolytic
1 100µF 16VW PC electrolytic
1 47µF RBLL electrolytic
1 10µF 16VW PC electrolytic
3 1µF MKT polyester
1 0.56µF MKT polyester (used
while adjusting sensitivity)
1 0.22µF MKT polyester
2 0.1µF MKT polyester
1 .015µF MKT polyester
Resistors (0.25W, 1%)
2 10MΩ
1 3.3MΩ
5 1MΩ
1 560kΩ
3 100kΩ
2 10kΩ
3 2.2kΩ
1 50kΩ (503) horizontal trim pot
(VR1)
Miscellaneous
Solder, neutral cure Silicone
sealant (roof & gutter type),
“body” for setting sensitivty etc.
*See text for alternatives
These two photos give a good idea of how
the whole lot goes together, especially the
“tricky bits” – securing the weight to the piezo
trans-ducer and splash-proofing the piezo buzzer with a flange from a light fitting and half a
ping-pong ball.
care with their orientation. Likewise,
the electrolytic capacitors (the MKT
types can be mounted either way
around). The accompanying capacitor
code table will help you in selecting
the value for each position.
Transistor Q1 and the PC stakes can
be inserted and soldered in position
now. Finally, trimpot VR1 can be
installed.
Piezo and weight
Remove the back from the piezo
transducer by prising the two halves
apart (the back is not used and can be
discarded). Attach the transducer in
place upside down on the PC board
using 10mm M3 screws and nuts.
For the weight attached to the transducer, we used a piece of brass water
tap plunger (the part that pushes the
valve down when you turn the tap
off), cut to 19mm long to clear the
back of the piezo buzzer when the
case is assembled.
Alternatively, you could use a
19mm long piece of 13-14mm diameter brass rod, or you could fashion
your own weight using a plumbers’
fitting such as a 12.5mm (1/2") brass
pipe cap (also known as a stop end),
cut to 19mm long and filling it with
lead or even solder.
This weight is glued to the piezo
element on the transducer using a
smear of silicone sealant between
Resistor Colour Codes
No. Value 4-Band Code (1%)
2 10MΩ brown black blue brown
1 3.3MΩ orange orange green brown
5 1MΩ
brown black green brown
1 560kΩ green blue yellow brown
3 100kΩ brown black yellow brown
2 10kΩ brown black orange brown
3 2.2kΩ red red red brown
5-Band Code (1%)
brown black black green brown
orange orange black yellow brown
brown black black yellow brown
green blue black orange brown
brown black black orange brown
brown black black red brown
red red black brown brown
the mating faces. Allow the sealant
to cure.
Also, while you have the silicone
sealant out, put a small dab in the
holes in the base of the case. This will
trap air inside the holes and provide
extra buoyancy.
The lid and cover
Cut a 25mm diameter hole in the
centre of the case lid, either with a
25mm hole-saw, or by first drilling
a series of small holes around the
required perimeter and removing this
piece then filing to shape. The piezo
siren should be a tight fit in the hole.
Next drill the holes for the two
switches using the front panel artwork
as a guide to their position.
We made our cover for the piezo
siren from a flange from a standard
Capacitor Codes
Value
1µF
0.56µF
0.22µF
0.1µF
.015µF
IEC code
105
564
224
104
153
EIA code
1u
560n
220n
100n
15n
SEPTEMBER 2000 17
(bayonet cap) light fitting. They’re all
much the same size.
The flange is placed with the larger
diameter end on the lid. File a couple
of small notches in this larger diameter end so that water can flow out
through these if some does enter. Test
that the notches are large enough for
water to flow out by placing the end
on a flat bench (eg, bathroom sink)
and pouring some water in. The water
should flow out leaving only a couple
of drops inside.
The smaller diameter end of the
bayonet flange requires brass wire
crosshairs to be placed symmetrically
across the opening so that the cut in
half table-tennis ball can be held in
place over the opening. We secured
the crosshairs in place by melting the
wires into the plastic flange using a
soldering iron.
The table tennis ball is cut in half
using a fine toothed hacksaw and
smoothed by rubbing the cut edge
on a sheet of fine glass-paper on a
flat surface. The half ball is secured
centrally over the wire crosshairs
and secured at these four points with
silicone sealant.
Attach the bayonet flange to the lid
with a smear of silicone sealant taking
care not to fill the water outlet notches
cut previously.
Four small L-shaped brackets are
required to hold the two AA cell
holders in place. We made ours from
some bits of chassis-mounting capacitor brackets but any small pieces of
metal would be fine. Exact size isn’t
critical – ours were about 7mm wide
and each leg was about 9-10mm long.
One of the legs on each bracket needs
to be drilled to accept the M3 screws.
Further support for the AA cell
holders is provided by the 25mm M3
screws mounted on the PC board.
When the silicone sealant has
dried, you can attach the front panel
label and secure the switches in place
along with their rubber boot covers.
Wire up as shown and insert the PC
board in place remembering to also
attach the right-angle battery holder
brackets under the corner mounting
screws.
Insert the cells and switch on power. Set S2 to the test alarm position.
Check that there is 6V between pins
4 and 8 of IC1 and between pins 1
and 8 of IC2. Pin 1 of IC1a should be
at around 3V after about 60 seconds
from power being switched on. Jerk
18 Silicon Chip
Full-sized artwork for the Pool
Alarm front
panel and PC
board. A photocopy of the front
panel makes a
handy drilling
template for the
case lid.
the box upward and check that the
siren sounds for about 50 seconds.
The “body” test
Testing in the swimming pool needs
to be done with the lid secured on
with its neoprene gasket in place.
That means you’ll need to remove
the lid each time you need to adjust
the sensitivity pot, VR1 but otherwise
you risk filling the alarm with water!
VR1 should be adjusted so that
the alarm will sound when a person
enters the pool but not so sensitive
that it is triggered with normal pool
water movements.
Adjustment of VR1 may be easier on
your ears if you temporarily replace
the 47µF capacitor on pin 6 of IC2 with
a 0.56µF capacitor. This will reduce
the alarm time to less than one second.
First, though, you'll need to find a
small volunteer “victim” (the smaller
the better) – but please, make sure
they can swim!
Get them to jump in, fall in and
even “ease” themselves into the pool,
setting the sensitivity pot (VR1) as low
as you can with the alarm triggering
reliably every time they go in.
To check that it will work with a
toddler, we don’t suggest throwing
one in(!) but perhaps a few bricks
wrapped in towels, weighing say
7-10kg, would be a reasonable approximation.
SC
|